Continuous steam generator

ABSTRACT

A continuous steam generator is provided. The continuous steam generator includes a combustion chamber having a number of burners for fossil fuels, downstream of which a vertical gas duct is mounted, on the hot gas side, in an upper region above a horizontal gas duct. The outside wall of the combustion chamber is formed, in a lower region, from evaporation tubes welded together in a gas-tight manner and mounted upstream of a water separator system, on the flow medium side, and in an upper region, from superheater tubes welded together in a gastight manner and mounted downstream of the water separator system on the flow medium side. The boundary between the regions of the evaporation tubes and the superheater tubes is essentially horizontal around the combustion chamber, in a region of the bottom of the horizontal gas duct.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2009/061239, filed Sep. 1, 2009 and claims the benefitthereof. The International Application claims the benefits of EuropeanPatent Office application No. 08015863.7 EP filed Sep. 9, 2008. All ofthe applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a once-though (“continuous”) steam generatorwith a combustion chamber having a number of burners for fossil fuel,downstream of which a vertical gas duct is mounted in an upper region onthe hot gas side above a horizontal gas duct, the surrounding wall ofthe combustion chamber being aimed, in a lower region, from evaporatortubes welded together in a gas-tight manner and mounted upstream of amoisture separation system on the flow medium side and, in an upperregion, from superheater tubes welded together in a gas-tight manner andmounted downstream of the moisture separation system on the flow mediumside.

BACKGROUND OF INVENTION

In a fossil fired steam generator, the energy of a fossil fuel is usedto produce superheated steam which in a power plant, for example, canthen be supplied to a steam turbine for power generation. Particularlyat the steam temperatures and pressures prevalent in a power plantenvironment, steam generators are normally implemented as water tubeboilers, i.e. the water supplied flows in a number of tubes which absorbenergy in the form of radiant heat of the burner flames and/or byconvection from the flue gas produced during combustion.

In the region of the burners, the steam generator tubes here usuallyconstitute the combustion chamber wall by being welded together in agas-tight manner. In other areas downstream of the combustion chamber onthe flue gas side, steam generator tubes disposed in the waste gas ductcan also be provided.

Fossil fired steam generators can be categorized on the basis of a largenumber of criteria: steam generators may in general be designed asnatural circulation, forced circulation or once-through steamgenerators. In a once-through steam generator, the heating of a numberof evaporator tubes results in complete evaporation of the flow mediumin the evaporator tubes in one pass. Once evaporated, the flow medium -usually water—is fed to superheater tubes downstream of the evaporatortubes where it is superheated. Strictly speaking, this description isvalid only at partial loads with subcritical pressure of water(P_(Kri)≈221 bar) in the evaporator—at which there is no temperature atwhich water and steam can be present simultaneously and therefore alsono phase separation is possible. However, for the sake of clarity, thisrepresentation will be used consistently in the following description.The position of the evaporation end point, i.e. the location at whichthe water content of the flow is completely evaporated, is variable anddependent on the operating mode. During full load operation of aonce-through steam generator of this kind, the evaporation end point is,for example, in an end region of the evaporator tubes, so that thesuperheating of the evaporated flow medium begins even in the evaporatortubes.

In contrast to a natural or forced circulation steam generator, aonce-through steam generator is not subject to pressure limiting, sothat it can be designed for main steam pressures well above the criticalpressure of water.

During light load operation or at startup, a once-through steamgenerator of this kind is usually operated with a minimum flow of flowmedium in the evaporator tubes in order to ensure reliable cooling ofthe evaporator tubes. For this purpose, particularly at low loads ofe.g. less than 40% of the design load, the pure mass flow through theevaporator is usually no longer sufficient to cool the evaporator tubes,so that an additional throughput of flow medium is superimposed in acirculatory manner on the flow medium passing through the evaporator.The operatively provided minimum flow of flow medium in the evaporatortubes is therefore not completely evaporated in the evaporator tubesduring startup or light load operation, so that unevaporated flowmedium, in particular a water-steam mixture, is still present at the endof the evaporator tubes during such an operating mode.

However, as the superheater tubes mounted downstream of the evaporatortubes of the once-through steam generator and usually only receivingflow medium after it has flowed through the combustion chamber walls arenot designed for a flow of unevaporated flow medium, once-through steamgenerators are generally designed such that water is reliably preventedfrom entering the superheater tubes even during startup or light loadoperation. To achieve this, the evaporator tubes are normally connectedto the superheater tubes mounted downstream thereof via a moistureseparation system. The moisture separator is used to separate thewater-steam mixture exiting the evaporator tubes during startup or lightload operation into water and steam. The steam is fed to the superheatertubes mounted downstream of the moisture separator, whereas theseparated water is returned to the evaporator tubes e.g. via acirculating pump or can be drained off via a flash tank.

Based on the flow direction of the gas stream, steam generators can alsobe subdivided, for example, into vertical and horizontal types. In thecase of fossil fired steam generators of vertical design, a distinctionis usually drawn between single-pass and two-pass boilers.

In the case of a single-pass or tower boiler, the flue gas produced bycombustion in the combustion chamber always flows vertically upward. Allthe heating surfaces disposed in the flue gas duct are above thecombustion chamber on the flue gas side. Tower boilers offer acomparatively simple design and simple control of the stresses producedby the thermal expansion of the tubes. In addition, all the heatingsurfaces of the evaporator tubes disposed in the flue gas duct arehorizontal and can therefore be completely dewatered, which may bedesirable in frost-prone environments.

In the case of the two-pass boiler, a horizontal gas duct leading into avertical gas duct is mounted in an upper region downstream of thecombustion chamber on the flue gas side. In said second vertical gasduct, the gas usually flows vertically from top to bottom. Therefore, inthe two-pass boiler, multiple flow baffling of the flue gas takes place.Advantages of this design are, for example, the lower installed heightand the resulting reduced manufacturing costs.

In a steam generator implemented as a two-pass boiler, the walls of thefirst pass, i.e. the combustion chamber, are usually implementedentirely as an evaporator. The moisture separation system downstream ofthe evaporator tubes on the flow medium side is accordingly disposed atthe upper end of the combustion chamber.

However, because of differences both in the geometry of the individualtubes and in the heating thereof, different mass flows and temperaturesof the flow medium occur in parallel tubes. These so-called asymmetriesmust be limited for the following reasons:

On the one hand, the evaporator heating surfaces must be sufficientlycooled over the entire load range of the steam generator. The mass flowrequired for cooling must be reliably supplied to each individual tube.In addition, the stresses occurring due to the thermal expansion of theindividual tubes must not exceed the permissible values between adjacenttubes. The temperatures of the flow medium must be limited both inabsolute terms and in terms of the difference with respect to theadjacent tubes, as otherwise damage to the combustion chamber wall couldarise.

To reduce temperature asymmetries in the evaporator tubes, mixing pointscan for example be installed in the combustion chamber walls configuredas evaporators. In this case the flow medium is diverted from theevaporator tubes, mixed and re-distributed to the other evaporatortubes. Such a system must be placed downstream of the mixing point foran even distribution of a water and steam mixture. A design of this kindaccordingly involves a high degree of technical complexity andconsiderably increases manufacturing costs.

SUMMARY OF INVENTION

The object of the invention is therefore to specify a once-through steamgenerator of the above-mentioned type which has a comparatively simpledesign while providing a particularly long service life.

This object is achieved according to the invention by disposing theboundary between the regions of the evaporator tubes and the superheatertubes in an essentially horizontally circumferential manner around thecombustion chamber in the region of the bottom of the horizontal gasduct.

The invention is based on the ideal that a simple design combined with acomparatively long service life would be achievable if comparativelyslight temperature asymmetries in the steam generator tubes wereachievable without an additional mixing point being disposed in theevaporator tubes.

The moisture separation system present in the steam generator alsocollects the water exiting the evaporator tubes in circulation mode andseparates it from the steam. In once-through operation, the incomingsteam is mixed and distributed to the superheater tubes locateddownstream on the flow medium side. This considerably reducestemperature asymmetries. Based on the knowledge that the moistureseparation system thus basically fulfils the function of a mixing point,by placing it lower down, e.g. in the region of the bottom of thehorizontal gas duct, this system can therefore be used as a mixing pointwithin the combustion chamber wall, without an additional mixing systembeing required.

In addition, this position of the moisture separation system means thatthe boundary between the regions of the evaporator tubes and thesuperheater tubes is disposed in an essentially horizontallycircumferential manner around the combustion chamber in the area of thebottom of the horizontal gas duct.

In an advantageous embodiment, the boundary between the regions of theevaporator tubes and the superheater tubes is disposed in an essentiallyhorizontally circumferential manner around the combustion chamber at thelevel of the edge formed by the surrounding wall and bottom of thehorizontal gas duct. By means of such an arrangement, all the combustionchamber tubes welded to the tubes of the walls of the horizontal gasduct are likewise designed as superheater tubes. In the existing designwith a combustion chamber formed entirely of evaporator tubes,evaporator and superheater tubes were welded in parallel at this point.This creates problems particularly for hot-starting of the steamgenerator, as the filling of the evaporator tubes with cold flow mediumproduces considerable temperature differences with respect to theunfilled superheater tubes. Disposing the moisture separation system atthe level of the edge formed by the combustion chamber wall and thebottom of the horizontal gas duct ensures that such a vertical interfaceno longer occurs and altogether more reliable operation of the steamgenerator can be achieved while at the same time providing acomparatively long service life.

In the case of two-pass steam generators, to improve gas flow, a sectionof the surrounding wall facing the vertical gas duct in inclined inwardbelow the horizontal gas duct, thereby forming, with the bottom of theadjacent horizontal gas duct, a projection extending into the combustionchamber. In steam generators of this kind, the boundary between theregions of the evaporator tubes and the superheater tubes isadvantageously disposed in an essentially horizontally circumferentialmanner around the combustion chamber directly above the projection.

In another advantageous embodiment, the bottom of the horizontal gasduct is formed of evaporator tubes welded together in a gas-tight mannerupstream of the moisture separation system on the flow medium side. Thebottom of the horizontal gas duct is actually suitable to be designed asan additional evaporator heating surface, as its tubes are not weldedparallel with the vertically tubed horizontal gas duct walls configuredas superheaters and therefore the stresses caused by differentialthermal expansion remain comparatively low.

The particular advantages of the invention are that dual use of themoisture separation system as a mixing point for reducing temperaturedifferences between parallel tubes is made possible by disposing theboundary between the regions of the evaporator tubes and the superheatertubes in an essentially horizontally circumferential manner around thecombustion chamber in the region of the bottom of the horizontal gasduct. In addition, one of the main disadvantages of the two-pass boiler,namely the vertical interface between wall heating surfaces configuredas evaporators and those configured as superheaters, are eliminated.Particularly for hot starting of the steam generator, during which hightemperature differences and stresses occur at said interface when theevaporator tubes are filled with comparatively cold flow medium,particularly reliable operation and a longer service life of the steamgenerator can be achieved by avoiding such stresses.

The lower positioning of the moisture separation system and therefore ofthe boundary between evaporator and superheater tubes in the combustionchamber also allows reduced superheating at the moisture separationsystem and altogether more material-conserving startup of the steamgenerator, which in turn results in a longer service life of the steamgenerator and enables less expensive materials to be used for themanufacture thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will now be explained ingreater detail with reference to the accompanying drawings in which thefigure schematically illustrates a once-through steam generator oftwo-pass design.

DETAILED DESCRIPTION OF INVENTION

The once-through steam generator 1 according to the figure comprises acombustion chamber 2 implemented as a vertical gas duct, downstream ofwhich a horizontal gas duct 6 is disposed in an upper region 4. Thehorizontal gas duct 6 is connected to another vertical gas duct 8.

In the lower region 10 of the combustion chamber 2 a number of burners(not shown in greater detail) are provided which combust liquid or solidfuel in the combustion chamber. The surrounding wall 12 of thecombustion chamber 2 is formed of steam generator tubes welded togetherin a gas-tight manner into which a flow medium—usually water—is pumpedby a pump 9 (not shown in greater detail), said flow medium being heatedby the heat produced by the burners. In the lower region 10 of thecombustion chamber 2, the steam generator tubes can be oriented eitherspirally or vertically. In the case of a spiral arrangement, althoughcomparatively greater design complexity is required, the resultingasymmetries between parallel tubes are comparatively lower than with avertically tubed combustion chamber 2.

The steam generator tubes in the lower part 10 of the combustion chamber2 are designed as evaporator tubes. The flow medium is first evaporatedtherein and fed via pipework 14 to a moisture separation system (notshown in greater detail). In the moisture separation system, not yetevaporated water is collected and drained off. The steam produced is fedinto the walls of the combustion chamber 2 and distributed to thesuperheater tubes disposed in the upper region 4 and in the walls of thehorizontal gas duct 6. Such removal of not yet evaporated water isparticularly necessary in startup mode when a larger amount of flowmedium must be pumped in for reliable cooling of the evaporator tubesthan can be evaporated in one evaporator tube pass.

To improve flue gas flow, the once-through steam generator 1 shown alsocomprises a projection 16 forming a direct transition to the bottom 18of the horizontal gas duct 6 and extending into the combustion chamber2. In addition, a grid 20 of further superheater tubes is disposed inthe transition region from the combustion chamber 2 to the horizontalgas duct 6 in the flue gas duct.

Particularly in the case of a vertically tubed combustion chamber 2,temperature differences between parallel evaporator tubes may now occurwhich can compromise the operation of the steam generator as the resultof differential thermal expansion. In order to achieve mixing of theflow medium from different tubes and therefore temperature equalizationwithout using additional components, the boundary 22 between evaporatortubes and superheater tubes is disposed directly above the projection 16at the level of the bottom 18 of the horizontal gas duct 6. The moistureseparation system therefore acts not only as a separator during startupoperation, but also as a mixing point in continuous operation, as theentire flow medium from the evaporator tubes is collected, mixed andredistributed to the superheater tubes in the moisture separationsystem.

As now both the upper part 4 of the combustion chamber 2 and the wallsof the horizontal gas duct 6 are configured as superheater tubes, thereis also no vertical interface in the region of the grid 20 betweenparallel welded evaporator and superheater tubes. Instead, only thelower part 10 of the combustion chamber 2 and the bottom 18 of thehorizontal gas duct are configured as evaporator tubes, as a result ofwhich only superheater tubes are welded together in parallel in thisarea.

1.-3. (canceled)
 4. A continuous steam generator, comprising: acombustion chamber including a plurality of burners for fossil fuel; avertical gas duct, disposed downstream of the combustion chamber andmounted in an upper region on a hot gas side above a horizontal gasduct; the horizontal duct; a plurality of evaporator tubes; a pluralityof superheater tubes; and a moisture separation system, wherein asurrounding wall of the combustion chamber is formed, in a lower region,from the plurality of evaporator tubes welded together in a gas-tightmanner and mounted upstream of a moisture separation system on a flowmedium side and in an upper region, from the plurality of superheatertubes welded together in a gas-tight manner and mounted downstream ofthe moisture separation system on the flow medium side, wherein part ofthe surrounding wall facing the vertical gas duct is inclined inwardbelow the horizontal gas duct, thereby forming with a first bottom ofthe adjacent horizontal gas duct a projection extending into thecombustion chamber, and wherein a boundary between the regions of theevaporator tubes and the plurality of superheater tubes is disposeddirectly above the projection in an essentially horizontallycircumferential manner around the combustion chamber in a region of thebottom of the horizontal gas duct.
 5. The continuous steam generator asclaimed in claim 5, wherein the boundary between the regions of theplurality of evaporator tubes and the plurality of superheater tubes isdisposed in an essentially horizontally circumferential manner aroundthe combustion chamber at a level of an edge formed by the surroundingwall and a second bottom of the horizontal gas duct.
 6. The continuoussteam generator as claimed in claim 5, wherein the second bottom of thehorizontal gas duct is formed of the plurality of evaporator tubeswelded together in a gas-tight manner upstream of the moistureseparation system on the flow medium side.